In a conventional water-supply system, raw water, which is taken in from rivers, is purified in a water purification plant. Clear water is supplied to customers via a plurality of distributing reservoirs. Meanwhile, in a sewage system, waste water, which is drained by customers, is conveyed to a sewage treatment plant via a plurality of pumping stations. The waste water is subjected to sewage treatment in the sewage treatment plant, and discharged to rivers, etc.
In this manner, the water-supply system and sewage system carry out their processes according to water demand by customers.
On the other hand, customers, in many cases, use water without being conscious of energy costs. It is empirically known that two peaks of water demand occur in the morning and evening in a day. The time zones of the morning and evening, at which the peaks of water demand occur, are time zones in which the power unit rate is high.
As described above, since the time zone in which the peak of water demand occurs overlaps the time zone in which the power unit rate is high, a great deal of water has to be treated in the water-supply system and sewage system in the time zones in which the power unit rate is high. Consequently, the cost of water treatment increases.